フェニルブタゾン
フェニルブタゾン（通称 **"bute"**）は、ピラゾロン系の古典的な非選択的非ステロイド性抗炎症薬（NSAID）です。 **臨床上のポイント：** 歴史的には強力な鎮痛、抗炎症、解熱作用のため多くの動物種で使用されてきましたが、現代の獣医療では、その使用はほぼ**馬の臨床**における筋骨格系の痛み（蹄葉炎、骨関節炎など）の管理に限定されています。 小動物（犬や猫）では、血液疾患、肝毒性、重度の胃腸潰瘍などの重篤な副作用のリスクが高いため、より安全なCOX-2選択的NSAID（カルプロフェンやメロキシカムなど）にほぼ置き換えられています。ヒトに対する骨髄毒性（再生不良性貧血）の安全上の懸念から、産業動物での使用は厳しく規制されています。
作用機序: Phenylbutazone acts primarily by inhibiting the **cyclooxygenase (COX)** enzymes (both COX-1 and COX-2), which prevents the conversion of arachidonic acid → **prostaglandins** and **thromboxanes**. * **↓ Prostaglandins** → Reduced inflammation, analgesia, and antipyresis. * **↓ Thromboxane A2** → Decreased platelet aggregation. **Pharmacological Note:** Phenylbutazone is metabolized in the liver to **oxyphenbutazone**, an active metabolite that contributes significantly to its prolonged efficacy (often >24 hours) despite a relatively short plasma half-life. The irreversible binding of phenylbutazone to cyclooxygenase also extends its duration of action. It also possesses mild uricosuric properties.
動物種別の用量
- General inflammation/pain · 4-8 mg/kg PO or 2-5 mg/kg IV · PO or IV · Extralabel use prohibited in female dairy cattle 20 months of age or older.
- General inflammation/pain · 10-20 mg/kg PO, then 2.5-5 mg/kg q24h or 10 mg/kg every 48 hours PO · PO · q24h or q48h · Extralabel use prohibited in female dairy cattle 20 months of age or older.
- To reduce pain or pyrexia associated with pleuropneumonia · 2.2-4.4 mg/kg · PO or IV · q12h · first week of treatment or longer
- General musculoskeletal inflammation · 1-2 grams per 454 kg (1000 lb.) horse IV, or 2-4 grams per 454 kg (1000 lb.) horse PO · IV or PO · daily · Limit IV administration to no more than 5 successive days · Do not exceed 4 grams/day. Use high end of dosage range initially, then titrate to lowest effective dose. Injection must be made slowly and with care.
- Adjunctive treatment of colic (to reduce endotoxic effects) · 2.2 mg/kg · PO or IV · twice daily
- Adjunctive treatment of laminitis · Initial dose of 4 grams, immediately decreased to 1 to 1.5 grams · PO or IV · twice daily · Dose for an average adult-sized horse. Lower dose is used to keep the horse comfortable, but not relieve pain to the extent the horse moves around excessively.
- Osteoarthritis · 2.2 mg/kg · PO · twice daily · Minimal dose to control pain.
- General inflammation/pain · 4 mg/kg · IV or PO · q24h
投与経路
禁忌
- Known hypersensitivity to phenylbutazone or other NSAIDs
- History of or preexisting hematologic or bone marrow abnormalities
- Preexisting gastrointestinal ulcers
- Food-producing animals (especially female dairy cattle 20 months of age or older)
- Lactating dairy cattle
- Pre-existing gastrointestinal ulceration
- Renal impairment
- Hepatic impairment
- Hematological disorders
- Use in food-producing animals (banned in many jurisdictions)
有害事象
- Gastrointestinal and oral erosions/ulcers
- Hypoalbuminemia (especially in foals/ponies)
- Renal papillary necrosis and azotemia
- Sodium and water retention (edema)
- Blood dyscrasias (agranulocytosis, aplastic anemia)
- Hepatotoxicity
- Severe tissue necrosis and sloughing (if injected IM or SC)
- CNS stimulation and seizures (if injected intracarotid)
- Gastrointestinal ulceration and bleeding
- Bone marrow suppression (aplastic anemia, leukopenia)
- Renal papillary necrosis
- Sodium and water retention
薬物相互作用
- Furosemide · Phenylbutazone may antagonize the increased renal blood flow effects caused by furosemide.
- Hepatotoxic Drugs · Concurrent administration may increase the chances of hepatotoxicity developing.
- NSAIDs · Concurrent use increases the potential for adverse GI and renal reactions. (Note: 'Stacking' with ketoprofen shows synergistic toxicity).
- Penicillamine · May increase the risk of hematologic and/or renal adverse reactions.
- Penicillin G · Phenylbutazone may increase the plasma half-life of penicillin G.
- Sulfonamides · Phenylbutazone could potentially displace sulfonamides from plasma proteins, increasing the risk for adverse effects.
- Warfarin · Phenylbutazone could potentially displace warfarin from plasma proteins, increasing the risk for bleeding.
- Corticosteroids · Increased risk of gastrointestinal ulceration · major
- Other NSAIDs · Increased risk of gastrointestinal ulceration and renal toxicity · major
- Highly protein-bound drugs (e.g., warfarin) · Displacement from plasma proteins leading to increased free drug concentrations · moderate
モニタリング
- Analgesic, anti-inflammatory, and antipyretic efficacy
- Complete blood counts (CBC) with chronic therapy (weekly early in therapy, biweekly later)
- Urinalysis and renal function parameters (serum creatinine/BUN) with chronic therapy
- Plasma protein determinations (especially in ponies, foals, and debilitated animals)
- Complete Blood Count (CBC)
- Renal function (BUN, Creatinine)
- Liver enzymes
- Clinical signs of GI bleeding (melena)
過量投与
Manifestations of acute overdosage include prompt respiratory or metabolic acidosis with compensatory hyperventilation, seizures, coma, and acute hypotensive crisis. * **Systemic Effects:** Clinical signs of renal failure (oliguric, proteinuria, hematuria), liver injury (hepatomegaly, jaundice), bone marrow depression, and severe GI ulceration/perforation may develop. * **Dogs:** Common signs include ataxia, seizures, tachycardia, trembling, tremors, vomiting, and tachypnea. Oral LD50 is 332 mg/kg. * **Horses:** Common signs include colic, anorexia, and ataxia. **Treatment:** Standard overdose procedures should be followed (e.g., empty gut following oral ingestion). Institute supportive treatment as necessary. Intravenous diazepam can be used to help control seizures. Monitor fluid therapy carefully, as phenylbutazone may cause fluid retention.
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